1. Field of the Invention
The present invention relates to a toner for developing electrostatic latent images in electrophotography, electrostatic recording, electrostatic printing, etc. and a charge control agent for negative charging capable of controlling or stabilizing the amount of charges of the toner, i.e., a negative charge control agent.
2. Description of the Prior Art
In copying machines etc. based on electrophotography, various dry or wet toners containing a coloring agent, a fixing resin and other substances are used to visualize the electrostatic latent image formed on the photoreceptor having a light-sensitive layer containing an inorganic or organic photoconductive substance.
The chargeability of such toners is one of the most important factors in electrostatic latent image developing systems. Thus, to appropriately control or stabilize the amount of charges of the toner, a charge control agent providing a positive or negative charge is often added to the toner.
Examples of conventional charge control agents providing a positive charge for toners include basic dyes, such as nigrosine dyes and triarylmethane dyes, and electron-donating substances, such as quaternary ammonium salts. Examples of conventional charge control agents providing a negative charge for toners include 2:1 metal complex salt azo dyes.
However, many of charge control agents having a dye structure are usually of complex structure, lacking stability, and are likely to decompose or deteriorate to lose their charge control property due to mechanical friction and impact, temperature and humidity changes, electric impact, light irradiation, etc. Such charge control agents lack versatility for color toner use, because they are colored.
Negative charge control agents offering a solution to these problems include chelate compounds with salicylic acid, alkylsalicylic acid, oxynaphthoic acid, salicylaldehyde, phthalic acid, or the like, as a ligand (e.g., Cr, Zn or Al complex of salicylic acid or alkylsalicylic acid, boron complex of salicylic acid, Cr complex of oxynaphthoic acid, Cr or Co complex of salicylaldehyde, Cr or Fe complex of phthalic acid). Although many of these charge control agents of complex structure are of light color, they are not necessarily satisfactory for use in color toners, their resin dispersibility and charge control property stability remain to be improved.
The object of the present invention is to provide a negative charge control agent which is excellent in charge control property, environmental resistance (charge control property stability to changes in temperature and humidity), storage stability (charge control property stability over time) and durability (charge control property stability during multiple repeated use of toner), which is versatile for use in various color toners and achromatic toners, and which is very safe to the human body, and a toner for developing electrostatic images that is excellent in charge control property, environmental resistance, storage stability and durability, and that can be used for various chromatic or achromatic colors.